In recent years, semiconductor light-emitting devices such as Light-emitting Diodes (LEDs) have been attracting attention as new light sources for lamps which can contribute to the prevention of global warming by saving energy because such LEDs provide a higher energy efficiency and have a longer product life than incandescent lamps and halogen lamps. Research and development on such LED lamps using LEDs as light sources is in progress.
It is known that optical output of an LED decreases and the product life becomes shorter as the temperature of the LED increases. For this reason, it is necessary for such an LED lamp to have an efficient heat transfer structure in order to suppress increase in the temperature. In view of this, various kinds of LED lamps having an efficient heat transfer structure have been conventionally proposed (for example, see Patent Literatures 1 to 3).
FIG. 12 and FIG. 13 are a cross sectional view and an exploded perspective view of a conventional LED lamp disclosed in Patent Literature 1, respectively. As shown in FIG. 12, in this conventional LED lamp, a through hole 228 and a first groove 232 allow communication between the circumferential part of an LED device 236 and the outside of the LED bulb 210. Thus, heat generated by the LED device 236 is transferred to the outside via the through hole 228 and the first groove 232.
On the other hand, Patent Literature 2 discloses a technique for suppressing increase in the temperature in an LED by providing a metal holder formed by integrating a circumferential side surface part which is exposed to the outside and a light source attachment part.
Further, Patent Literature 3 discloses forming a fin for increasing a heat transfer effect on the outer circumferential surface of an LED lamp.